Hydropic chorionic villi are found in hydropic abortuses, partial hydatidiform moles (PM), and complete hydatidiform moles. Partial and complete moles have the potential for persistent trophoblastic disease. The vast majority of partial moles are triploid and generally follow a benign clinical course. Complete moles are diploid and distant metastasis and choriocarcinoma may develop. The authors determined the nuclear ploidy by flow cytometry of 31 placentas, 19 of which appeared hydropic either on obstetric ultrasonography or gross examination. Of ten complete moles classified by histologic criteria, ten were diploid, whereas five of seven histologically classified PM were triploid. The remaining two cases classified as PM were diploid; one most likely represented a regressing complete mole; the other a hydropic abortus. All 14 control placentas were diploid. In all cases in which karyotypic analysis was performed, the flow cytometric determination of ploidy was confirmed. It was concluded that DNA flow cytometric analysis is a rapid, accurate, and cost-effective means for assaying nuclear ploidy in these tissues, and as such, offers an informative supplement to the histological interpretation of hydropic placentas.